Reactions of Ammonia on Stoichiometric and Reduced TiO2(001) Single Crystal Surfaces

The reaction of NH3 on the surface of the {011}-faceted structure of the TiO2(001) single crystal is studied and compared to that on the O-defected surface. Temperature-programmed desorption (TPD) conducted after NH3 adsorption at 300 K shows only molecular desorption at 340 K. Modeling of TPD signals as a function of surface coverage indicated that the activation energy, Ed, and pre-exponential factor, veff, decrease with increasing coverage. Near zero surface coverage, Ed was found to be equal to 92 kJ/mol and veff to be close to 1013 /s. Both parameters decreased to ∼52 kJ/mol and ∼107 /s at saturation coverage. The decrease is due to a repulsive interaction of adsorbed NH3 molecules on the surface. Computing of the TPD results show that saturation is obtained at 1/2 monolayer coverage (referred to Ti atoms). Both the amount and shape of NH3 peak change on the reduced (Ar+-sputtered) surfaces. The desorption peak at 340 K is considerably attenuated on mildly reduced surfaces (TiO∼1.9) and has totally d...